The use of power semiconductor devices including MOSFETs (Metal Oxide Silicon Field Effect Transistors) is rapidly increasing not only in the market of large-current high-breakdown-voltage switching power sources but also, especially in recent years, in the market of power-saving switching for mobile communication apparatuses such as notebook PCs. The power semiconductor devices are used in power-management circuits, safety circuits for lithium ion batteries, or the like. Accordingly, the power semiconductor devices are designed to achieve a low drive voltage that enables the power semiconductor devices to be directly driven with a battery voltage, a low on-resistance, a reduction in capacitance between the gate and the drain, or the like.
For instance, a technology is known in which an n-channel MOSFET with a trench-gate structure is made to have a low on-resistance by narrowing the trench pitch. In a MOSFET with a narrowed trench pitch, among an n+-type source layer and a p+-type contact layer both connected to a source electrode, the p+-type contact layer has a difficulty in securing a large enough contact area for the connection.
For this reason, the potential of a p-type base layer, which is connected to the p+-type contact layer, is difficult to fix to the source potential. Thus, there is a problem that an avalanche resistance of the semiconductor device is lowered.
Avalanche breakdown occurs, for instance, when a surge voltage caused by induced electromotive force is larger than a breakdown voltage of the MOSFET in a switching-OFF operation in an inductive load. Here, the avalanche resistance is the ability to withstand the avalanche breakdown.
Suppose a case where the potential of the p-type base layer is not completely fixed to the source potential. In this case, when holes generated by the avalanche breakdown flow as the electric current through the source electrode, the holes pass under the n+-type source layer. Hence, a potential difference is generated between the source electrode and the p-type base layer, and an npn-type bipolar transistor that is parasitic in the MOSFET is turned ON. Consequently, concentration of the current occurs, and the MOSFET becomes more likely to be broken down.